The aim of this research is to compare preoperative, initial and later postoperative psychological capacities of patients with hemispherectomy for infantile hemiplegia and for brain tumor to provide data on the principles underlying the organization, disorganization and reorganization of cerebral functions in the development of the brain. Accumulating data have demonstrated systematic differences between effects of left vs. right hemispherectomy for neoplastic or traumatic lesions in adults. Left "dominant" hemispherectomy results in marked persisting impairment of speech, reading, verbal reasoning and auditory memory, but comprehension of speech is relatively spared, and nonverbal visual ideational, construction and memory functions are intact. Right hemispherectomy results in a reciprocal syndrome of selective impairment of nonlanguage functions, but language and verbal reasoning capacities are intact. The development of superior language, verbal and nonverbal reasoning capacities following left hemispherectomy for epileptogenic lesions incurred at birth in this research reflects the "functional plasticity" of the infant brain. Systematic studies of effects of hemispherectomy for lesions incurred at different stages in the development of the brain will provide data for assessing 1) the nature, rate, and extent to which the "functional plasticity" of the infant brain diminishes with advancing age; 2) the extent to which each hemisphere participates in certain language and nonlanguage functions and specializes in others as the brain matures; and 3) the limitations and principles underlying reorganization of cerebral functions following removals of either hemisphere for lesions incurred at different stages in the development of the brain.